Semi-solid metal processing refers to a metal forming processing method which is achieved by using the semi-solid temperature regions during the metal transition from solid state to liquid state, or from liquid state to solid state. In early 1970s, the semi-solid processing technology was firstly proposed by those researchers in Massachusetts Institute of Technology, USA. This technology employs two typical characterizations of non-dendritic and semi-solid slurry, breaks the traditional dendrite solidification mode, and has the unique advantages such as small deformation resistance, high material utilization rate, easy to implement automation, prone to form novel processing technology, etc., thus attracting highly attentions from the researchers in various countries, and the products prepared by semi-solid processing and application thereof are also developed rapidly.
However, so far, the studies of the semi-solid processing technology are mainly focused on the low melting point alloy system, such as aluminum alloy, magnesium alloy and the like, and the microstructure of the prepared alloy has relatively large and coarse grains. At the same time, the microstructures of fine grains such as ultra-fine crystalline, nanocrystalline, etc., can not be obtained by using the conventional semi-solid processing methods (such as rheocasting, rheoforging, thixoforging, etc.), let alone prepare the bimodal microstructure wherein any two size grains selected from the three structures of fine crystalline, ultra-fine crystalline, and nanocrystalline. In fact, the research results show that the presence of the bimodal microstructure in iron, titanium, aluminum and alloy thereof will generally significantly improve the comprehensive performances of the bulk material. In addition, the preparation of the slurry and billet material in the conventional semi-solid processing method is complicated, and it is hard to prepare the semi-solid slurry of the high melting point alloy, thus limiting the research and application of the semi-solid processing in the high melting point alloy system such as titanium alloy, nickel alloy, and the like.
In recent years, a series of the titanium alloy materials with a bimodal structure of nanocrystalline matrix/amorphous matrix+micron-sized ductile β-Ti dendrite have been obtained by the research stuffs using the copper mold casting rapid solidification method. During the deformation, the nanocrystalline matrix/amorphous matrix contributes to the ultra high strength, and the ductile β-Ti dendrite contributes to the high plasticity of the material, with a fracture strength of more than 2000 MPa, and a fracture strain of more than 10%. Thereafter, more and more the high strength and toughness alloy systems having such microstructure (comprising Fe-based, Zr-based, Ti-based, etc.) are reported. The key point of the preparation method lies in that the alloy components are elaborately designed and the solidification conditions of the alloy melt are precisely controlled [G. He, J. Eckert, W. Loser, and L. Schultz, Nat. Mater. 2, 33 (2003)], wherein during the solidification process, the suitable temperature maintaining regions are selected so that the β-Ti phase is preferentially nucleated and grown, and formed dendrites, then the remaining alloy melt is rapidly cooled to form nanocrystalline or amorphous matrix. However, there are two disadvantages present in this method, one is that as the five-components composition is prone to form intermetallic compounds, the enhancement effect of the dendrite is counteracted, the ductility of the material is deteriorated, so that the ranges of the components which can form the nanocrystalline matrix/amorphous matrix+ductile β-Ti dendrite are relatively narrow; the other one is that during the copper mold casting process, the cooling rate is limited, so that the prepared high strength and toughness titanium alloy with the bimodal structure has a size of several millimeters (less than 4 mm) in general. The abovementioned factors become a bottleneck for practical application of these high strength and toughness titanium alloys with the bimodal structure.
As an alternative forming technology, powder metallurgy technology has the characteristics for examples, the prepared material has uniform composition, the material utilization rate is high, such technology is a near-net-shape forming technology, and the like, and it is easy to prepare a high strength and toughness alloy with a ultra-fine crystalline/nanocrystalline structure, which is commonly used to prepare a relatively large size and complicated shape alloy parts. As for the combination of the semi-solid processing technology and the powder metallurgy technology (such as powder forging, powder extrusion, powder rolling, etc.), generally, the low melting point matrix alloy particles and the high melting point reinforcing particles are mixed, then heated to the semi-solid region of the matrix alloy, stirred, and further processed and formed to prepare a composite material. However, as the inherent defects are present in the additional reinforcing phase in the composite material (i.e, the poor wettability with the matrix alloy), and it is hard for the semi-solid powder metallurgy method to ensure the homogenous distribution of the second phase in the matrix, there are a substantial room for improvement in the performances of the composite material prepared by combining the semi-solid processing and the powder metallurgy technology.
In view of this, if a novel microstructure such as nanocrystalline, ultra-fine crystalline, fine crystalline or even bimodal structure can be obtained by using semi-solid processing technology in the high melting point alloy system such as titanium alloy and the like, it will provide a novel preparation method for developing a novel high performance and high melting point alloy material, and the engineering parts therefrom for industrial application.